Deionized water is needed for many industrial applications, as desalination, industrial waste purification, and electronics industry processes. Typically, deionization is carried out using ion exchange resins followed by reverse osmosis. The ion exchange resins are expensive and may cause problems such as growth of bacteria on the resins, and introduction of sodium ions in the water if cation exchange resins are used, which may be undesirable. Anion exchange resins can become corrosive to plumbing and fixtures, and may result in elevated levels of copper, iron, or lead in the water in addition to shortening the life of the fixtures. Reverse osmosis processes are energy-intensive, requiring high water pressures to move water through clogged membranes. Distillation is a way to obtain water without some of these problems but it is very energy and labor intensive and impractical in most cases.
Deionization of water has been carried out electrolytically utilizing carbon electrodes made of particulate carbon bonded with a binder. The disadvantage of this method is that such electrodes are not durable. The binder degrades resulting in disintegration of the electrode. The presence of binder also reduces the conductivity of the electrode.
More recently, U.S. Pat. No. 5,425,858 discloses a method of removing ions from water by electrolysis using a relatively large number of electrodes made of carbon bonded to a support plate made of titanium, which is expensive. The carbon is made by mixing resin with a fiber mat and then carbonizing. It is bound to the titanium plate with a polymer binder. Because the carbon is present as a supported material, the effective amount of carbon, which is in essence, the functioning electrode, is not large. Therefore a large number of electrodes must be used to accomplish any significant amount of deionization. Another disadvantage of this method is that the distance between electrodes must be maintained at a critical distance of about 0.5 mm for deionization to take place efficiently. To achieve this distance, the plates must be engineered to be perfectly flat, which is expensive, and plumbing is critical. The water follows a serpentine path between the electrodes, which requires complex plumbing, and throughput suffers as a result of the long flow path.
U.S. Pat. Nos. 5,415,768 and 5,620,597 relate to deionization systems fabricated from activated carbon fibers. The use of activated carbon fibers is not very advantageous from economic as well as performance point of view. The activated carbon fibers are derived from carbon fibers which are expensive due to the inherently expensive manufacturing process. These fibers are then subjected to further activation process at high temperature which increases the cost of the fibers even more. A device containing these fibers becomes prohibitively expensive. From the performance point of view also the fiber geometry is not desirable. The commercial carbon and activated carbon fibers have a very fine diameter (ten microns). On activation (which causes pore formation in fibers) the fibers become very weak and can easily break during normal handling. As a result of this weakness, a fiber strand (containing several fiber) contains multiple broken ends along its length. These broken ends create discontinuities, causing problems in uniform current flow. The fibers have to be packaged in a strand containing several hundred fibers because of the low strength. This prevents access of liquid to the interior fibers in the strand due to surface tension forces and so majority of the fibers do not participate in the process. The fibers strands do not have good conductivity because of the broken ends as well as presence of a large number of individual fibers in the strand, as opposed to a monolithic structure. This results in loss of power and hence inefficient operation of the device. Use of a conductive support is necessary for the fibers because of their flexibility. The support has to be fabricated out of a very expensive metal such as titanium or a relatively inexpensive material such as graphite foil. The graphite foil material is a graphite material which is conductive and flexible, however, because of the need to make it flexible, it has to be very thin and as a result is very fragile. It is extremely difficult to attach any electrical connections to it and make a robust device for practical applications, which would survive any stresses such as would be applied by flowing water for any length of time.
Use of pressed activated carbon washers containing binders becomes a problem from the point of view of conductivity due to nonconductive binders, as well as preferred conduction path due to random particle to particle contact. Long term durability of these forms is questionable.
Consequently there still exists a need in various applications for cost effective, efficient, and simpler methods for removal of ions from water.
The present invention provides such an ion removal system.